Why will my flash not fire above a certain shutter speed?

Because above that speed the shutter never has the whole frame open at once, so a single burst lights only part of it. The limit belongs to the camera and is published per body; the flash cannot raise it.

A focal plane shutter is two curtains. Below the sync speed they part completely before closing; above it, they travel as a moving slit.

What the frame sees

A burst of flash lasts a fraction of the exposure. If the whole sensor is uncovered at that instant, the whole frame is lit. If a slit is travelling across it, the light lands only where the slit has reached, and the rest of the frame stays dark — a hard-edged band, not a gradient.

Where the figure comes from

The camera, and the camera alone. Makers publish it per body, and it is the shutter design that sets it. No flash raises it, and no trigger raises it.

The mode that works past it

Two titles in this catalogue print the abbreviation for high-speed sync, and one of them names a ceiling of 1/8000 s. The mode replaces one burst with a pulse train, which fills the whole travel of the slit — at the cost of output, since the same energy is spread across a longer period.

What to do without it

Keep the shutter at or below the published limit and control the exposure elsewhere: close the aperture, or cut the ambient light with a neutral density filter. This catalogue keeps those filters as their own family, and the reason they exist in a flash workflow is this limit.

Where this page and the maker’s own manual disagree, the manual decides. Every focal length, aperture, guide number, stop of stabilisation, load rating and sealing claim on this page is reported from the manufacturer’s own documentation, with its unit written as published, and named as such — none of it is measured here. An engraved figure is reported as the maker engraves it, and is never restated in another convention, because the two are not the same number. And where two light sources are set beside each other: a guide number and an energy in watt-seconds describe different quantities and cannot be compared with one another. A guide number depends on the sensitivity and the reference coverage the maker chose before printing it; a watt-second figure describes the energy a capacitor holds before the tube fires. Neither is the light that reaches a subject, which depends on the modifier, the distance and the room.